A conventional electronic key system for a vehicle is made up of an in-vehicle device that is mounted in a vehicle, and a mobile unit that is carried by a user of the vehicle. This electronic key system executes the control of the lock/unlock of doors or engine starting when authentication of the mobile unit is approved via radio communication between the in-vehicle device and the mobile unit, even if a mechanical key is not operated.
When the mobile unit of this electronic key system is lost, it is not easy to find the mobile unit. Therefore, JP 2005-220728A proposes a mobile unit locating device that is used when the user wants to locate the missing mobile unit. However, even when this mobile unit locating device is used, it is not frequently found at what specific location the mobile unit is at although it is possible to roughly know a certain area in which the mobile unit exists.
More specifically, for example, when the mobile unit is placed in a pocket of clothing or in a bag of a user, even if the area in which the mobile unit exists can be specified by using the mobile unit locating device, the user cannot sometimes find where the mobile unit exists within the area. In this case, when there are a lot of possible locations in which the mobile unit may be placed, user must search all those locations for the mobile unit, and therefore cannot find the mobile unit easily. Also, for example, when the mobile unit is lost outdoors at night, even if the area in which the mobile unit exists can be specified by using the mobile unit locating device, where the mobile unit is located cannot be found in a dark location with no light.